Fuel efficiency and performance of an internal combustion engine may be improved by varying the timing of intake and/or exhaust valves. Adjusting valve timing may allow additional air to be inducted into a cylinder, thereby increasing the engine performance. Furthermore, the engine valve timing may be adjusted so that engine pumping losses may be reduced during certain operating conditions. For example, a throttle located upstream of an intake manifold and intake valve timing may be adjusted so that intake manifold pressure may be increased without increasing the cylinder air charge. As a result, the engine pumping work may be decreased while a desired engine torque is maintained.
Engine fuel efficiency may also be improved by stopping an engine during periods where the operator is not requesting torque or where the torque request is less than a predetermined amount. Then, when the torque demand increases or when there is a desire to charge a battery, for example, the engine may be restarted to supply the desired torque. By stopping the engine during these conditions, the overall fuel efficiency of a vehicle may be increased during a drive cycle.
One method to control a variable event valvetrain during an engine start is described in U.S. Pat. No. 5,765,514. This method provides for closing the intake and exhaust valves after the ignition switch is turned on and then the starter is used to crank the engine. If a signal pulse representing crankshaft rotation through 720 degrees has been generated, an injection sequence for each cylinder and a crankshaft position sequence are set. The fuel injection sequence for the cylinders is initialized when a first crankshaft pulse is generated and after producing a first signal pulse that represents crankshaft rotation through 720 degrees. The injection sequence and crankshaft position sequence correspond to the position of each cylinder, whereby the opening/closing timing of each intake valve and exhaust valve can be controlled. The cylinders are set to the exhaust stroke, suction stroke, compression stroke, and explosion stroke, respectively.
The above-mentioned method can also have several disadvantages. Namely, by putting the intake and exhaust valves in a neutral position when the ignition switch is off, air may be allowed to flow through the engine and exhaust system. As the engine and after treatment system cool, air may be drawn into the after treatment system by convective cooling. That is, heated gases in the engine air path can seek to flow to a lower energy state. These gases may be replaced by ambient air that is drawn into the after treatment system by the temperature induced gas flow. Air flowing through the exhaust system can disturb the amount of oxygen stored in the catalyst, thereby permitting excess oxygen to occupy catalyst sites that might otherwise be available for conversion of undesirable gases. Consequently, vehicle tail pipe emissions may increase when the engine is restarted because fewer catalyst sites may be available to convert the exhaust gasses. Furthermore, once excess oxygen is stored in the catalyst the engine air-fuel ratio may be enriched so that the excess oxygen is consumed by using it to oxidize CO and HC's. Although this practice may increase the catalyst conversion efficiency, it may also increase fuel consumption.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method to control engine valves during stopping and starting that offers substantial improvements.